Cathedral ceiling exterior insulation retrofit
BobHr
| Posted in General Questions on
Climate zone 5A . I need to replace my roof and want to add exterior insulation over a cathedral ceiling.
I am pretty sure there is poly in the ceiling and am worried about trapped moisture. How should a person handle a retrofit when there is drywall, poly, fiberglass batt insulation, roof deck and shingles.
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Are you going to be converting a vented roof to an unvented?
Yes....isnt that my only choice when adding rigid insualtion
Well, most roofs are vented. If your current roof was not vented, then maybe you wouldn't be making much of a change by adding the foam. On the other hand, if you are sealing the vent opening and adding enough foam over the sheathing, maybe the sheathing will stay warm enough to avoid condensation risk. And, you could use something like felt-faced polyiso with sleepers over it, or one of the vented nailbase panels, and maintain some drying potential to the outside.
Is the ceiling drywall or wood?
Robert,
As David has implied, the trickiest part of this job will be to do a good job sealing air entry at the soffit vents and air exit and the ridge vents. You'll need excellent access to these areas. You'll probably need to use spray polyurethane foam for this job -- perhaps a two-component kit.
The presence of interior polyethylene is unfortunate, but it's not a deal-killer. As long as your roof sheathing looks good when you strip your old roofing, you'll probably be fine.
However, if you see signs of moisture after you strip the roofing -- rot or mold -- all bets are off. At that point, you'll need Plan B.
I was wondering about removing the bottom row of sheathing and removing the poly that is accessible At that point I could air seal the top plates and put blocking in. That would a little drying to the interior.
The home is 23 years old. Would/could there be an air space above the fiberglass for the current venting.
Would the drywall support dense pack cellulose. I dont know if I am going to have the money for that as I had bypass and will be off work for 2 months.
Robert,
Q. "Could there be an air space above the fiberglass for the current venting?"
A. Yes, there could. The only way to tell, though, is to inspect the roof assembly.
Regardless of whether there is an air space, you'll still need to seal the air entry points at the eaves near the soffit vents, as well as the air exit points near the ridge vents.
If there IS an air space above the pre-existing fiberglass (and there probably is) it's worth blowing cellulose over the fiberglass into those cavities to limit the thermal bypass & convection potential, even after doing your best to seal up the venting.
It's pretty quick & easy to drill ~1" holes every ~4-6' and fill until the blower stalls at each, rather than snaking in a dense-packing hose. With the exterior foam the seasonal humidity cycling of the cellulose will be low enough that settling over time will not become an issue even at ~2.5lbs density- you don't need the full-on dense-pack treatment here. With a foam over-blanket above the roof deck it's akin to moving to a warmer-drier climate, from the point of view of cellulose settling.
If you don't fully fill the rafter bays there is still some potential for convection to deposit moisture at the eaves (the cold end of the rafter bay), even with the exterior foam, but filling it will cellulose reduces that convection loop by 3 orders of magnitude or better.